In the production of glass articles such as bottles, molten glass is produced in a glass furnace, and then flows through a forehearth in a continuous molten glass stream to a feeder bowl from which the molten glass is fed in gobs into a glassware-forming section. A typical forehearth comprises a refractory trough for carrying the molten glass covered with an insulated roof.
It is recognized in the art of glassmaking that it is difficult to maintain the temperature of the glass homogeneous throughout its cross-section while being carried from the melting furnace to the feeder bowl. The glass tends to cool more rapidly at the outside edges of the glass stream due to the cooling effect of the sidewalls of the trough. For this reason, heating means such as gas burners, submersible electrodes, or the like have been provided in the sidewalls of the forehearth to heat the glass. Cooling air may also be blown into the forehearth either transversely or longitudinally of the direction of flow of the molten glass to help control the temperature of the glass stream. With the proper rates of heating and cooling, homogeneity of the glass temperature across the stream of glass can be improved.
Accordingly, glass forehearth structures have been proposed which include heaters positioned in the sidewalls of the forehearth structure to heat the glass stream along the longitudinal edges of the stream. One such structure is described in the aforesaid commonly assigned application Ser. No. 559,160, now U.S. Pat. No. 4,552,579, the disclosure thereof being incorporated herein by reference. The forehearth described in Ser. No. 559,160 comprises a trough having a roof thereover; a pair of spaced projections extending downwardly from the roof to define a central channel over the central portion of the stream of glass, and side channels over respective side portions of the stream of glass. The roof has a reduced thickness in the portion over the central channel. An enclosed cooling channel extends longitudinally of the forehearth over the areas of reduced thickness. The forehearth includes means at the sidewalls thereof for directing heat against the spaced projections extending downwardly from the roof. The heat is deflected from the projections to thereby heat the side portions of the glass stream.
In the embodiment of the forehearth shown in the drawing of the application; the heating is accomplished by combustion of gases in gas burners. However, it is stated that heating can be accomplished electrically by insertion of electrodes spaced at proper intervals along the length of the forehearth into the glass stream so that the current flow results in preferential heating of the glass along the edges longitudinally. With electrodes submerged in the glass, voltage is applied between electrodes, resulting in a flow of electric current through the glass and dissipation of energy by I.sup.2 R loss in the glass. This method has a disadvantage in that it is difficult or impossible to control the path of the electric current so that energy cannot easily be added in the portion of the glass desired. Furthermore, the electric current flows preferentially through the hottest glass, which tends to exaggerate temperature non-uniformity in the glass, which is generally the opposite of what is desired. In addition, it has been found that electric heating by this method frequently results in creation of blisters in the glass as a result of localized overheating, electrochemical cell creation, or other origin. Further, the electrodes must be essentially permanently installed, so that they are inaccessible for easy removal or replacement in the event of breakage.
It has also been proposed to electrically heat the forehearth with silicon carbide elements installed beneath the forehearth roof. However, the heating cannot be concentrated at the forehearth side as with gas burners since the elements must span the forehearth roof. Forced cooling is also difficult or impossible to arrange. Silicon carbide has also been found to be an undesirable material in the presence of glass since it gradually deteriorates and particles can fall into the glass, causing defects in the glass articles to be formed.